I got it from Terence, a GameFAQs user who hacks games to get raw data. His coverage of FFVII game mechanics is unreal. The only thing I've seen him do with FFIX was get the stat formulae from hacking, and he also did a test where the ExII became available again after 12 hours had passed by letting the timer run for two and half years (under test conditions of course, on an emulator with the FPS jacked waaaay up).
I know a lot about battle mechanics in FFIX just from doing my own tests, but any percentage based attacks and such (Stop, Thievery, Odin, etc...), I haven't found a way to test those properly. Physical attacks and magical attacks though, I know how damage is calculated for those.